This invention relates to flame-retardant resin compositions. More particularly, this invention relates to flame retardant, silicone resin compositions suitable for use as translucent coating materials for a variety of substrates, including fabrics formed from glass fibers, and to translucent fibrous substrates coated with these resin compositions.
In recent years there has been an increasing interest in new construction materials for the exterior portions of buildings which can be heated using solar energy. In addition to transmitting at least a portion of the incident sunlight into the interior of the structure, these materials should be sufficiently durable to withstand prolonged exposure to the elements of weather and comply with the applicable building codes.
Panels of plate glass meet all of the foregoing criteria, and have heretofore been one of the preferred materials for solar heated buildings, particularly agricultural greenhouses. Glass panels do have a number of disadvantages, among which are their rigidity, brittleness and the requirement for a strong supporting framework due to their considerable weight. This framework must also provide for the thermally induced contraction and expansion exhibited by glass.
One alternative that avoids some of the foregoing disadvantages associated with using plate glass as a structural material is to replace the glass with sheets of relatively strong, high-melting organic polymers such as polymethyl methacrylate. While these polymers are lighter than glass, they do have other serious deficiencies, including relatively low resistance to abrasion and degradation caused by heat, weathering, and ultraviolet radiation. If incorporated into a permanent structure, panels formed from these polymers would have to be replaced periodically, thereby substantially increasing the maintenance costs of the structure.
The physical and chemical properties of organosilicone resins would make them one of the preferred materials for the exterior of solar heated buildings, were it not for the fact that these resins are usually not strong enough to form self-supporting films. When the resins are coated onto a suitable substrate such as a fabric formed from glass fibers, the resultant material is flexible and translucent or transparent, depending upon the closeness of the weave in the fabric. This material could be used for the exteriors of tension- or air-supported structures if it were made sufficiently flame retardant to comply with current regulations pertaining to the flammability of construction materials while still retaining at least a substantial fraction of the light transmittance exhibited by the unmodified resin-coated fabric.
Most silicone resins exhibit excellent resistance to degradation during exposure to temperatures as high as 500.degree. C., however these resins will burn if they are placed in a flame. The flammability of silicone resins has delayed their widespread use in products such as coatings for construction materials which must comply with regulations requiring that the final product be either non-burning or at least self-extinguishing within a specified time interval once the source of the flame has been removed. This requirement has provided the incentive to search for effective flame retarding agents suitable for use with organosilicone materials in general and polyorganosiloxane resins in particular.
The prior art discloses numerous classes of compounds that will function as flame retardants for organic polymers. A partial listing of commercially available materials from representative classes of flame retardants is contained in a text entitled "Flammability Handbook for Plastics" by Carlos J. Hilado (Technomic Publishing Company, Westport, Conn., 1974). Useful flame retardants can be divided into several classes, based on their chemical composition. These classes include compounds containing one or more of chlorine, bromine, phosphorus, nitrogen, antimony, boron and arsenic. Of these classes of compounds, the most effective flame retardants for organic polymers have been found to be those containing bromine, chlorine, and/or phosphorus. Antimony compounds are usually relatively poor flame retardants, however they have been shown to interact synergistically with many organic halogen-containing compounds.
It is well known that the mechanism by which flame retardation is achieved varies with the particular compound and polymeric substrate, as do the adverse effects which flame retardants have on the physical properties of the substrate and the concentration level at which these adverse effects become apparent. Once this level is exceeded, the physical properties of the substrate often degrade to the extent that the utility of the substrate is destroyed for all practical purposes.
Flame retarding agents that have been proposed for use with elastomeric polysiloxanes include platinum and fumed titanium dioxide, optionally in combination with carbon black (U.S. Pat. No. 3,635,874 to T. L. Laur and P. Lamont), platinum compounds in combination with conventional fillers (U.S. Pat. No. 3,514,424 to M. G. Noble and J. R. Brower), finely divided copper or copper compounds in a vinyl-containing polymer (U.S. Pat. No. 2,891,033 to R. M. Savage), powdered copper or copper compounds and various chlorinated organic compounds (U.S. Pat. No. 3,154,515 to C. A. Berridge and British Patent No. 1,399,172) and a platinum compound in combination with triphenyl phosphite (U.S. Pat. No. 3,734,877 to G. Christie). The combination of carbon black and platinum as a flame retardant for a specified class of elastomeric polydiorganosiloxanes is disclosed in U.S. Pat. No. 3,734,881 to R. A. Shingledecker.
Experimental data in a subsequent portion of this specification demonstrate that platinum is a virtually ineffective flame retardant for coatings of polysiloxane resins.
Using a combination of powdered glass and mica to flame retard various coatings, including polysiloxane resins, that are applied on fiber substrates is taught in Japanese Patent Publication No. 53/16883.
Flame retardant polyorganosiloxane elastomers containing a brominated diphenyl ether as the flame retarding agent and chalk as a filler are disclosed in German Offenlegungsschrift No. 3,002,867.
U.S. Pat. No. 2,684,349 teaches that finely divided glass, preferably in the form of glass frit having an average particle size below 15 microns, can be employed in combination with conventional fillers, such as silica, calcium carbonate and titanium dioxide, to improve the flame retardant properties of elastomeric polyorganosiloxanes employed as coating materials for various substrates, including cloth woven from glass fibers. Particles larger than about 15 microns are undesirable since they may adversely affect desirable properties of the polyorganosiloxane.
While it is true that halogen-containing compounds such as the brominated diphenyl ethers disclosed in the aforementioned German Offenlegungsschrift No. 3,002,867 and fillers such as chalk and titanium dioxide impart useful levels of flame retardancy to polyorganosiloxane elastomers and resins, the specific formulations disclosed in the prior art employ these compounds at concentration levels that would render the material into which they are incorporated virtually opaque, and therefore useless as a light-transmitting coating material.
It is therefore an objective of this invention to define a class of flame retardants that can be incorporated into polyorganosiloxane resins at concentration levels that impart a desired level of flame retardancy while retaining a useful fraction of the light transmittance exhibited by films of the unmodified resin.
It is also an objective of this invention to provide flame-retardant organosilicone resin compositions that can be applied as translucent coatings on a variety of substrates, including glass fibers.
As used in this specification and the accompanying claims, an organosilicone resin is defined as translucent if a film of the resin containing an additive will transmit at least 30% of the incident sunlight transmitted by a film of the same thickness formed from the same resin without the additive.
It has now been found that the foregoing objectives can be achieved using a combination of a bromine- or chlorine-containing organic compound and finely divided glass particles. The composition may also include compounds such as antimony trioxide, which are known to interact synergistically with halogen-containing flame retardants.